This invention relates to a method for stabilizing the vibration frequency of a tuning fork-type quartz crystal oscillator particularly designed to vibrate within a low frequency range from about 1 KHz to about 1 MHz. A tuning fork-type quartz crystal oscillator whose crystal element vibrates within such a low frequency range may be exemplified by one adapted to vibrate generally at 32.768 KHz for a quartz crystal electric watch.
Such prior art tuning fork-type quartz crystal oscillators include a type constructed as shown in FIG. 1. This type is provided with a base member 11 on which a pair of rod electrodes 12a and 12b concurrently serving as external electrodes are mounted at a predetermined interval and extend upward and downward of the base member 11. A substantially U-shaped tuning fork-type quartz crystal element 13 of the undermentioned configuration is disposed at a predetermined position above the base member 1 between the paired rod electrodes 12a and 12b.
The quartz crystal element 13 comprises a pair of arms 13a and 13b formed with substantially the same dimensions, i.e., cross section and length, and a common connecting section 13c connecting the mutually facing ends of the paired crystal arms 13a and 13b.
Under this arrangement, the quartz crystal element 13 is fitted by a pair of flexible lead wires 14a and 14b made of, e.g., phosphor bronze to the paired rod electrodes 12a and 12b, by soldering a pair of nodal points 15a and 15b which are present on a pair of crystal electrodes previously evaporated on the surface of the quartz crystal element 13 to the corresponding rod electrodes 12a and 12b through the respective lead wires 14a and 14b. Thus, after fixing the crystal element 13 to the base member 11 as described above, an electroconductive metal cover 16 having a substantially U-shaped cross section and acting as an electric shield is hermetically sealed on the base member 11 as shown by a dotted line in FIG. 1. The paired flexible lead wires 14a and 14b, the paired rod electrodes 12a and 12b, the base member 11 and the metal cover 16 collectively function as a holding mechanism 17 for supporting the crystal element 13.
However, with the prior art tuning fork-type quartz crystal oscillator constructed as shown in FIG. 1, the quartz crystal element 13 is fitted only to the paired rod electrodes 12a and 12b by flexible lead wires 14a and 14b. Consequently, said prior art crystal oscillator not only exhibits low resistance to external mechanical vibrations and shocks but also tends to cause the supporting position of the crystal element 13 relative to the holding mechanism 17 to be deviated during a long use, and in consequence has the disadvantage that is unadapted for, e.g., an electric watch the vibration frequency of whose crystal oscillator requires to be kept at the highest possible precision and stability for a long period of time.
Accordingly, there has recently been proposed a tuning fork-type quartz crystal oscillator as shown in FIG. 2 which is intended to improve the crystal oscillator of FIG. 1.
The crystal oscillator of FIG. 2 is almost equivalent to that of FIG. 1, excepting that a holding mechanism 171 for supporting the quartz crystal element 13 is provided with a fine supporting rod 18 made of appropriate elastic material such as phosphor bronze or gold and disposed between substantially the center of the bottom of the aforesaid common crystal connecting section 13c and that upper surface portion of the base member 11 which faces the supporting rod 18, in addition to the same construction as the holding mechanism 17 including the paired flexible lead wires 14a and 14b, base member 11 and the cover 16. Therefore, parts of FIG. 2 corresponding to those of FIG. 1 are designated by the same symbols and the description thereof is omitted.
The crystal oscillator constructed as shown in FIG. 2 can allow the quartz crystal element 13 to be fixed to its holding mechanism 171 in a more stable state than in the crystal oscillator of FIG. 1 due to addition of the aforesaid supporting rod 18 to the holding mechanism 17 of FIG. 1, enabling the vibration frequency of the crystal oscillator to be kept at a considerably high stability for a long period of time.
However, the crystal oscillator of FIG. 2 still has the disadvantages that:
a. the supporting rod 18 is fitted only to substantially the center of the bottom of the common connecting section 13c of the paired crystal arms 13a and 13b by point contact, and in consequence not only prevents the crystal element 13 attached to the base member 11 from presenting sufficient resistance to external mechanical vibrations and shocks, but also requires high degree of technique and sense in the process of fitting the supporting rod 18 to the crystal element 13;
b. the supporting rod 18 also tends to remain deformed during long use, leading to deviations in the vibration frequency of the cyrstal oscillator; and
c. since a tuning fork-type quartz crystal oscillator is ordinarily designed mechanically to vibrate in the so-called tuning fork-type mode in which its paired crystal arms oscillate the preset number of times in the opposite directions to almost the same degree (see FIGS. 3 and 5), part of the vibration energy of the paired crystal arms is transmitted to the common connecting section thereof, causing part of the aforesaid transmitted vibration energy to leak to the outside from the crystal oscillator, with the failure to prevent any slight deviation of the vibration frequency of the oscillator when it is incorporated in such a desired instrument as an electric crystal watch, even if the vibration frequency of the oscillator is previously defined with any high precision.
The reason for the above-mentioned drawbacks accompanying the crystal oscillator constructed as shown in FIG. 2 originates with the fact that a shearing force A--A' acting in such opposite directions as to increase the width W of the common crystal connecting section 13c starting with a connecting point 19 of the supporting rod 18 and the bottom of common crystal connecting section 13c, a shearing force B--B' acting in the same direction decreasing the length of the supporting rod 18, and a rotational moment M--M' acting in the direction causing the common crystal connecting sections to rotate in the direction of the supporting rod 18 all these three forces are applied to the bottom of the common crystal connecting section 13c, when the paired crystal arms 13a and 13b vibrate in the above-mentioned manner. The aforesaid shearing forces A--A' and B--B' and rotational moment M--M' also adversely affect the base member 11, rendering the vibration frequency of the crystal oscillator unstable.
Accordingly, the crystal oscillator constructed as shown in FIG. 2 is also unadapted to be applied to such an instrument as an electric crystal watch whose crystal oscillator should have its vibration frequency maintained at the highest possible precision and stability for a long period of time. It is, therefore, the object of this invention to provide a tuning fork-type quartz crystal oscillator capable of keeping is vibration frequency at the highest possible precision and stability for a long period of time under any environment.